1. Field of the Invention
The present invention generally relates to optical viewing apparatus and, more particularly, to those systems which permit the viewing and recording of vertical, horizontal and rotary eye movements.
2. The evaluation of eye movements in patients is an important diagnostic tool in the clinical investigation of certain organic disorders. The techniques which are disclosed by the prior art have been generally designated as electronystagmography (ENG), electro-oculograph (EOG), photoelectronostagmography (PENG) and Frenzel lenses.
The systems defined as ENG and EOG differ only in the time constant of the recording equipment. ENG uses a time constant (AC recording) while EOG does not (DC recording). ENG and EOG use the inherent potential difference between the cornea and the retina of the eye (the corneal-retinal potential) as a means for recording eye movements. This technique is based on the fact that the eye acts as a dipole with the cornea being positively charged relative to the retina. The dipole axis is said to correspond to the visual axis. By placing electrodes at the skin surface adjacent to the eye orbit, the potential changes resulting from eye movements are recorded on appropriate types of polygraph instruments. The AC recording system of ENG mainly records eye movements, whereas eye positions as well as eye movements are recorded using the DC mode of EOG. There are inherent problems in both of these systems. Skin potential responses are in the same general frequency band as the eye movements, and in some subjects are of greater amplitude than the corneal-retinal potential. A second problem arises since the polarization of the skin electrodes causes baseline shifts in the recorded corneal-retinal potential, which may prevent the recording of the resulting eye movements.
Some of the electronic problems associated with ENG and EOG recording methods are overcome with the system defined as PENG. This technique, using infrared sensitive photoelectric cells mounted on goggles is able to detect the difference in reflected light off the sclera relative to the iris. The difference in reflectivity during eye movement is translated into an electrical signal which can be recorded on a polygraph instrument similar to that used with ENG or EOG methods. With this technique, the problem of electropolarization or skin potential artifacts is eliminated as no connection is made to the skin. However, eyelid tremor and blinking still cause artifacts which complicate the analysis of the polygraph traces. The major disadvantage of PENG is that testing must be done with the eyes open and in some cases, the infrared light used was of a lower wave length and could be seen by the patient after a period of dark adaptation. Thus, optic fixation when present will reduce or inhibit the nystagmus. The problem of resolving eye position with PENG is the same as that encountered with electro-techniques.
Frenzel lenses are glasses which are plus 20 diopter lenses, having a built-in light source. They allow for direct observation of eye movements because the lenses act as magnifiers. The problems which are inherent in this system is that they do not provide total elimination of optic fixation, and they do not give a permanent record of eye movement responses. The major disadvantage of all the other techniques is it is impossible to record rotary eye movements, i.e., movement of the eye about the visual axis.
For electrode methods, it is because the dipole is in the plane of the visual axis and therefore there is no change in its recording at the electrode sites during a rotary eye movement. And for PENG, the reflectivity of the sclera (the white of the eye) and the iris (the colored center portion) does not change relative to the infrared sensors during rotary eye movement.
The present invention substantially resolves the problems which are inherent in systems disclosed by the prior art. The present invention employs an infrared radiation source which illuminates the patient's eyes with non-visible infrared radiation. The infrared radiation source is affixed directly to spherical eye pieces mounted over the patient's eyes or the emitted radiation can be transmitted through the use of a fiberoptic light bundle. The goggle eye pieces are fully enclosed to preclude the entry of any ambient light. The interior surface of the eye pieces are coated with a reflective surface which will substantially reflect and disperse the emitted infrared energy. The infrared radiation reflected from the eye of the subject is monitored by a coupled video sensor. The reflected radiation is monitored directly or can be transmitted through a fiberoptic light bundle to a relay lens of a video camera which is adapted for infrared viewing. The output of the video camera is coupled to a monitor and recorder to provide for subsequent analysis. Since the present invention is fully operational even when the patient moves his head, eye movements following head movements can be properly recorded. In addition, since ambient light is precluded and the impinging radiation is outside of the band width of visible light, the problem of optic fixation is fully resolved.